P1262: EZH2 INHIBITORS MEDIATE PLATINUM RESISTANCE BY ENHANCED EFFLUX
Authors: E. Groß, R.-A. Hilger, J. Wehde, F. L. Schümann, C. Rohde, E. Willscher, J. Lützkendorf, L. P. Müller, B. Edemir, T. Müller, M. Binder, G. Posern, T. Weber
Journal: HemaSphere (2022)

Background: T-cell neoplasms are a heterogeneous and rare group of lymphomas with unfavorable prognosis. Polychemotherapy has cure rates of only 30 %. Consequently, new strategies of therapies are needed (Vose et al, J Clin Oncol, 2008, 26(25): 4124-30). EZH2 is the catalytic subunit of the Polycomb Repressive Complex 2 and responsible for the trimethylation of Lysine 27 of Histone 3 (H3K27me3). The protein is overexpressed in several tumor entities including T-cell neoplasms and leads to epigenetic dysregulation by H3K27me3 (McCabe et al, PNAS, 2012, 109(8): 2989–2994). Thus, EZH2 inhibition (EZH2i) might be a promising target and is in clinical evaluation in B- and T-cell malignancies showing favorable results (Duan et al, J Hematol Oncol, 2020, 13(1): 104)). Aims: We aimed to investigate EZH2i in a characterized panel of Peripheral T-Cell Lymphoma (PTCL) and T-cell leukemia cell lines in combinational therapy with widely used salvage chemotherapeutics. Methods: The T-cell leukemia and PTCL cell lines Jurkat, Loucy, HH, and DERL-2 were characterized regarding the canonical EZH2 signaling. They were treated with the EZH2 specific inhibitors GSK126 or EPZ6438 in combination with the common second line chemotherapy platinum analogue Oxaliplatin. The change in cytotoxic effects under pharmacological EZH2i or EZH2 Knockdown was evaluated. Cell viability was analyzed by Resazurin assay after 72 h of incubation. Target genes of chemical EZH2i that are responsible for the change in Oxaliplatin sensitivity were uncovered by RNA sequencing and validated by qPCR and western blotting. The effect of inhibiting EZH2 downstream targets on Oxaliplatin sensitivity modulation were analyzed. Results: EZH2i led to a drastic increase in Oxaliplatin resistance in all tested cell lines regardless of cell type or EZH2 and H3K27 methylation status. This result could not be accomplished by EZH2 knockdown. RNA sequencing revealed an increase of expression in the transporters ABCG1 and ABCG2 by EZH2i that are associated with chemotherapy resistance. In addition, SRE binding factors SREBF1 and SREBF2 were overexpressed under EZH2i. Coherently, SREBP1 is known to activate ABCG1 transcription (Ecker et al, Biochem Biophys Res Commun, 2007, 352(3): 805-11). Under EZH2i, platinum was less present in the cells proven by inductive coupled plasma mass spectrometry. The EZH2i effect on Oxaliplatin resistance was lowered by additional inhibition of SRE binding proteins or ABC transporters. Summary/Conclusion: We have shown that EZH2i is not suitable in combination with the common chemotherapeutic Oxaliplatin in T-cell neoplasms. EZH2i leads to increased expression of SREBF1 and SREBF2 as well as ABCG1 and ABCG2. These transcriptional changes lead to increased Oxaliplatin efflux resulting in increased resistance. This effect was an EZH2-independent off-target effect of chemical EZH2i. Our data do not support the combination of platinum derivates and EZH2i in PTCL.